The tumorigenicity of 5-azacytidine in the male Fischer rat.

5-Azacytidine was administered to young adult male Fischer rats. Tumors were found in 31 out of 70 rats that had received 5-azacytidine and survived 18 months from the start of the experiment. Several rats had multiple primary tumors. In the rats that were tested for complete carcinogenicity a variety of tumor types was found. These included acute leukemia and malignant reticuloendotheliosis, and tumors of the testis, skin, and bronchus. No hepatic tumors were found in the group that was tested for hepatic tumor initiation. Hepatocellular carcinomas were found only in the group that was examined for hepatic tumor promotion by receiving a prior initiating dose of diethylnitrosamine. No tumors were found in the age controls. Thus, in these initial experiments, 5-azacytidine appeared to be a complete carcinogen, inducing tumors in several organs, and a tumor promoter but not a complete carcinogen for the liver.